Engines may be configured with boosting devices, such as turbochargers or superchargers, to increase mass airflow into a combustion chamber. Turbochargers and superchargers compress intake air entering the engine using an intake compressor. While a turbocharger includes a compressor that is mechanically driven by an exhaust turbine, an electric supercharger includes a compressor that is electrically driven by a motor. In some engine systems, one or more intake charging devices may be staged in series or parallel to decrease engine boost response times.
Electric superchargers may need to be intermittently diagnosed. If the supercharger is degraded, engine performance and fuel economy may be affected. In one example, an electric supercharger may become degraded when components within the supercharger are not able to spin as freely as designed, resulting in a drop in the supercharger's efficiency. This may cause the supercharger motor to draw more electrical power to realize the same relative increase in manifold pressure for a given manifold air flow.
One example approach for diagnosing a supercharger is shown by Morizane et al. in U.S. Pat. No. 8,033,272. Therein, Morizane presents a method for diagnosing an electric supercharger that includes actuating a series of valves, responsive to reaching a temperature threshold after the engine is turned off, and then delivering power to the supercharger motor when the engine is stopped and characterizing the state of the supercharger by detecting an amount of airflow through an intake passage until the airflow reaches a peak value. Other approaches include modeling the compressor to determine its output power, modeling the compressor motor to determine the electrical power consumed by the compressor, and comparing the power consumed with the output power. If the difference between the electrical power consumed and output power exceeds a threshold, degradation of the electric supercharger is indicated.
However the inventors herein have identified potential issues with such approaches. As one example, operating the supercharger when the engine is stopped may deplete the charge level of the battery during conditions when the battery is unable to be recharged. As another example, the operation of the supercharger may cause concern to an operator as an engine system continues to operate after the operator has requested the engine off. As yet another example, the approaches may be computationally complex and may require the coordinated actuation of numerous components, which may each be prone to their own degradation. Additionally, due to constantly changing manifold air flow and pressure when an engine is operating, computational models of the compressor and motor may not be able to appropriately characterize variation of other electrical loads on the vehicle system and those impacts on the diagnostic outcome. As a result, the modeled electrical usage of the motor may differ from the actual usage, resulting in an unreliable diagnosis. As such, monitoring the electrical efficiency of the supercharger may be difficult due to the lack of a direct measurement of its electrical power usage.
In one example, some of the above issues may be addressed by a method for a boosted engine comprising: during engine idling conditions, opening a bypass coupling an electric supercharger to an intake passage; operating an electric motor of the supercharger with step-wise incremented output; and indicating degradation of the supercharger based on a change in each of supercharger compressor speed and vehicle current following the operating. In this way, measureable vehicle current, such as an alternator current, may be advantageously used to assess the electrical efficiency of the supercharger, even as engine electrical loads vary, without affecting engine performance.
As an example, during engine idling conditions, a bypass valve coupling a supercharger compressor to an intake passage may be closed enabling a larger portion of intake airflow to be directed through the compressor. A control signal commanded to an electric motor driving the supercharger compressor may then be increased in a step-wise manner. For example, a first motor speed may be commanded, following which a change in compressor speed and a total vehicle current (such as an alternator current) may be measured. Then, a second motor speed, higher than the first motor speed, may be commanded, following which the change in compressor speed and total vehicle current may again be measured. At each step, the measured change in compressor speed and current may be compared to an expected change, the expected change based on the commanded (e.g., first or second) motor speed. Since the engine electrical demand may change due to engine components other than the electric motor, the change in total vehicle current cannot be assumed to be due exclusively to the supercharger. However, by applying multiple step-changes in supercharger command, and measuring the change in total vehicle current for each step, the current changes due to the supercharger may be isolated. Responsive to the measured change in compressor speed and/or vehicle current being lower than expected, supercharger degradation may be inferred. Further, a degree of degradation may be determined based on the difference and appropriate mitigating steps may be taken in accordance.
In this way, by monitoring changes in the supercharger compressor speed and vehicle current responsive to incremental changes in operation of the electrical motor coupled to the compressor, potential degradation of the electric supercharger efficiency may be reliably identified. The technical effect of using the engine alternator current is that a directly measurable vehicle electrical power usage may be correlated with the electrical power usage of the motor and the electrical efficiency of the supercharger, instead of relying on modeled behavior which may be differ from the actual behavior due to instantaneous vehicle operating conditions. By opportunistically performing the supercharger diagnostics during engine idling conditions, the diagnostics may be completed without inconvenience to the operator and without requiring extensive and complex computation. Additionally, by correlating changes in a command delivered to the supercharger compressor motor with changes in alternator current, the electrical efficiency may be diagnosed while appropriately accounting for any electrical load on the vehicle system, separate from the electrical compressor. Consequently, a more robust diagnostic of an electric supercharger may be provided. By timely diagnosing and addressed supercharger electrical efficiency, component life may be extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.